With the increasing prevalence of digitally operated electronic audio and video equipment such as camcorders and video tape recorder (VTRs) which are equipped for remote control, computer accessories which allow a home computer to control such audio/video equipment have been introduced to the market. However, the input/output interfaces, data formats and protocols used in home computers and the interfacing circuitry, data formats and protocols used in home video and audio equipment generally are not compatible. Therefore, elaborate hardware and software interfaces have been designed for converting the data formats and protocol of computers to the data formats and protocol of the various audio/video devices. Prior art interfaces for allowing communication between home computers and audio/video equipment typically have comprised an intermediate microprocessor with a standardized serial input/output port on the computer side and custom hardware compatible with the audio/video equipment on the audio/video side. The microprocessor includes software for converting data received from the serial port of the computer into a data format compatible with the audio/video equipment and for converting data received from the audio/video port to a data format compatible with the computer protocol. The cost of the intermediate microprocessor device including the various interfaces is substantial and may exceed the cost of the audio/video equipment which is to be controlled.
A standardized serial interface commonly utilized in home computers is the RS (Recommended Standard) -422 interface published by the Electonic Industries Association. For instance, the RS-422 standard interface is used in the Apple II and MacIntosh series of computers manufactured by Apple Computers, Inc. of Cupertino, Calif. Much literature is available on the RS-422 standard such as S. H. Stone, Microcomputer Interfacing, Addison-Wesley Publishing Co., 1982, pp161-178, which is incorporated by reference. Therefore, the standard will be only briefly described herein and reference should be made to other sources, including the above text, for a more detailed description of the RS-422 standard interface.
The RS-422 standard is called an asynchronous protocol because there is no organized timing with respect to separate bytes of data. However, data transmission in the RS-422 standard is synchronous with respect to the individual bits within a single data byte. In the RS-422 standard serial interface, data is transmitted in packages (or characters) of ten or eleven bits comprising a start bit, eight bits of data and one or two stop bits. FIG. 1 illustrates the bit timing for the serial transmission of a byte of data using the RS-422 serial standard. The idle state 12 is a logic level 1. Each byte transmitted serially on the asynchronous link begins with a start bit 14 at logic level 0. The data byte is then transmitted in the following eight bit intervals with the least significant bit transmitted first and the most significant bit transmitted last. Therefore, as shown in FIG. 1, for example, the data byte 00110101 is transmitted in the second through ninth time intervals in the reverse order, 10101100. The data is then followed by 1, 11/2 or 2 stop bits set to logic level 1. The transmission baud rate (and thus the bit length can be any of a number of possibilities.
The start and stop bits, in addition to identifying the beginning and ending of each byte, also permit the receiver of the byte to resychronize its clock to each new character. Since the RS-422 serial interface is asynchronous, a character can begin at an arbitrary time. Therefore, the receiving device must determine when the character begin with sufficient accuracy so that it is able to sample the next 10 or 11 bits properly. Thus, the standard practice is for the receiver to wait 1/2 of an interval after receipt of the leading edge of the start bit to sample the data input line. This causes the receiver to sample the start bit at its midpoint. Thereafter, the clock of the receiving device causes it to sample the input line at periods equal to the bit interval so as to sample the following 9 or 10 bits at their midpoints. In this manner, if the clock in the receiving device and the clock in the transmitting device are within a five percent skew error of each other, the character will be received correctly. For instance, if the clock in the receiving device is running faster than the clock in the transmitting device, the receiving device will sample the data input line successively earlier in each bit interval over the following 10 or 11 sampling points. If the receiving clock is running less than 5% faster than the transmitting clock, the 11th sample point will still occur after the leading edge of the 11th transmitted bit. Much greater accuracy than 5% is easily achieved by present technology.
An RS-422 serial interface comprises two separate but mating interfacing portions providing complementary input and output lines as shown in FIG. 2. The two portions will be termed herein the computer portion 11 and the telecom portion 13. These terms are used because the RS-422 standard was originally developed for interfacing computers and other devices over telephone lines. However, it should be understood that these are used merely to provide distinction between the two portions and are not intended to limit the applicability of the RS-422 standard serial interface.
Some of the available lines will be described with respect to the computer portion 11 and it should be understood that the telecom portion 13 has complementary lines bearing the same names but having the opposite direction, i.e. the line on the telecom portion bearing the same name as a line that is an input line on the computer portion is an output line on the telecom portion and vice versa.
The RS-422 standard serial interface utilizes differentially driven wire pairs for each signal. Therefore, in the following description, each signal line described actually comprises two differentially driven signal pairs. Further, since the lines are differentially driven, data can be placed on the "data send" lines so that it is received at the receiving device either unaltered or inverted depending on how the "data send" lines are coupled. The computer portion of a RS-422 standard interface includes an input "receive data" bit 15A for receiving serial data from a telecom portion; a "data send" bit 17A for transmitting serial data to a telecom device; a "request to send" bit 19A for indicating to a telecom portion equipped device that data is ready to be sent; a "clear to send" bit 21A for receiving, from another device having the telecom portion, a signal in response to the "request to send" signal indicating that the other device is ready to receive data; a "data mode" bit 23A for receiving a signal from another device indicating that it wishes to write to the computer; and a "receive ready" bit 25A for responding to the "data mode" signal when the computer is ready to receive data. The signal lines of the telecom portion are designated with the same reference numerals as the complementary lines on the computer portion but designated with the letter B rather than A.
If the computer and RS-422 interface do not use compatible logic levels, the signal levels used within the computer, typically TTL or ECL, are converted to the logic levels used by the RS-422 immediately before transmission.
Audio and video equipment such as VTRs, tape decks, camcorders, etc. typically do not utilize the RS-422 standard interface but use custom integrated circuits to accomplish interfacing with other devices. For instance, video and audio equipment sold by Sony Corporation of America, of Paramus, N.J., utilizes custom ICs to carry out interfacing between devices via one of three protocols, referred to hereinafter as Control-L, Control-S wired and Control-S infrared. The Control-L and Control-S wired protocols allow most transport and control functions in many consumer camcorders and VTRs to be remotely controlled over a wire connection by another device. Control-S infrared allows the same type of functions to be controlled by another device via infrared transmissions. Control-S wired and Control-S infrared utilize the same data format, while Control-L utilizes a different format. Each of these three protocols will be described in detail in the following sections.
It is an object of the present invention to provide a low cost interface for allowing a home computer to communicate with and control home audio and video equipment.
It is a further object of the present invention to provide a low cost device which provides interfacing between an RS-422 standard computer serial interface and a consumer electronic product which uses Control-L, Control-S wired, or Control-S infrared protocol.
It is yet another object of the present invention to provide a method and apparatus for converting RS-422 standard protocol to Control-L, Control-S wired, and Control-S infrared protocol.